Thermostatic expansion valve for refrigeration system

ABSTRACT

A thermostatic expansion valve for use in a refrigeration system of the vapor-compression type. Valve actuation is made responsive to the combined effect of evaporator coil temperature and pressure as independently monitored through separate control inputs. A slotted ball is pivotally rotated in its seat in a direction determined by the net pressure differential applied to the control inputs to present a gradually and uniformly varying flow orifice profile. One control input monitors internal coil pressure directly while the other control input senses the gas pressure produced within a thermostatic bulb in contact with the coil exterior. Calibration means are provided to balance the net pressure differential under desired steady-state cooling conditions and position the valve accordingly.

United States Patent Coyle, Sr.

[54] THERMOSTATIC EXPANSION VALVE FOR REFRIGERATION SYSTEM [72] Inventor: Roy M. Coyle, Sr., Glen Ellen, Calif.

[73] Assignee: Norman J. Wray, Glen Ellen, Calif.

; a part interest [22] Filed: Aug. 19,1971 I [21] Appl.No.: 173,211

[451 Oct. 10, 1972 Primary ExaminerW illiam F. ODea Assistant Examiner-P. D. Ferguson AttomeyTownsend and Townsend [57] ABSTRACT A thermostatic expansion valve for use in a refrigeration system of the vapor-compression type. Valve actuation is made responsive to the combined effect of evaporator coil temperature and pressure as independently monitored through separate control inputs. A slotted ball is pivotally rotated in its seat in a direction determined by the net pressure difierential applied to the control inputs to present a gradually and uniformly varying flow orifice profile. One control input monitors internal coil pressure directly while the other control input senses the gas pressure produced within a thermostatic bulb in contact with the coil exterior. Calibration means are provided to balance the net pressure differential under desired steady-state cooling conditions and position the valve accordingly.

11 Claims, 8 Drawing Figures Pmmnncrw m2 3.696.628

v/ g so 5| 5o 6 FIG 1 500- JNVENT 5 ROY M. CO

FIG ATTORNEYS THERMOSTATIC EXPANSION VALVE FOR REFRIGERATION SYSTEM The present invention relates to refrigeration systems of the vapor-compression type. More specifically, the invention is directed to a thermostatic expansion valve employing a slotted ball whose movement is responsive to the combined effect of evaporator coil temperature and pressure each of which is monitored through an independent control input.

Refrigeration systems typically include an expansion valve connected between an evaporator cooling coil on the one hand and a compressor, condenser and liquid reservoir on the other. The function of the valve is to admit controlled quantities of refrigerant introduced at its input in liquid form under pressure into the evaporator coil. Due to a relatively lower pressure in the evaporator coil produced by connecting the coil outlet to the suction side of the compressor, the liquid expands and evaporates. The evaporator coils are located in the space to be cooled and the change of state of the refrigerant from liquid to vapor within the coils absorbs heat from the surrounding air through the coil walls. The compressor withdraws the refrigerant vapor from the cooling coils through a suction line and delivers high pressure vapor to the condenser where it is returned to a liquid state producing a heat outflow through the walls of the condenser section. The liquid refrigerant is then collected in a receiver tank and resupplied to the valve inlet.

In conventional systems of this type, actuation of the expansion valve is typically directly responsive either to the evaporator pressure within the cooling coils or to some thermal index, such as room temperature, coil temperature or some combination of temperatures, either of which is used as a control parameter determining the amount of refrigerant admitted to the coil. However, it is a limitation of prior art devices that monitoring either coil pressure or temperature alone or alternatively inferring one from the other tends to produce overcompensation in the cooling rate and consequently requires correction to achieve a desired operating temperature. A typical cooling characteristic for a prior art device which illustrates change in temperature versus time is shown in FIG. 1, which depicts the overshoot and compensation effect of conventional systems. The result of such a problem is temperature band or range within which the system operates, shown as A in FIG. 1.

In applications where operating temperature is critical as for example in the refrigerated storage of perishable food, a cooling system is required which maintains temperature fluctuations within closely controlled limits, and systems characterized by a wide operating temperature band are unsatisfactory for this purpose.

It is also desirable to utilize a valve structure which allows a uniform and gradual flow rate transition from fully closed to fully open position. Prior art valve structures such as needle valves or solid ball valves have not satisfactorily achieved this uniform metering-function and tend to be either fully on or fully off.

The present invention provides a temperature stabilizing refrigeration system utilizing a novel slotted ball valve. Actuation of the valve is responsive to the difference between the pressure monitored by each of its two control inputs. One of these control inputs directly senses internal coil pressure, while the pressure at the other input is indicative of coil temperature. The pressures sensed at each of the two control inputs are applied to opposite ends of a solid linkage member which pivots the slotted ball open or closed in its seat depending on which control input senses the greater pressure.

The valve operation can be thought of as being responsive primarily to coil temperature with a modifying or supplementary coil pressure control to avoid coil overloading or flooding and to provide more positive valve actuation at the beginning and end of the on cycle.

Accordingly, it is an object of the present invention to provide a thermostatic expansion valve for a refrigeration system which is responsive to the combined effect of the temperature and the pressure of the evaporator cooling coil measured independently and thus coordinates coil conditions to achieve optimum control of coil loading.

Another object of the present invention is to provide a differential control expansion valve which is opened or closed in response to the pressure difference across its two control inputs, one of which monitors coil pressure directly and the other of which reflects coil temperature.

A further object of the present invention is to provide a refrigerant metering valve of the above character which regulates the flow of the refrigerant to the expansion coil gradually and uniformly through a flow rate range from fully closed to fully open position.

It is still another object of the present invention to provide a thermostatic expansion valve which avoids coil flooding and enables more positive shut-off at the end of the compressor on cycle and more positive opening at the beginning of the next cycle.

It is a still further object of the present invention to accomplish the above objectives by means of a slotted ball valve which is pivoted in its seat to present a flow orifice of gradually varying cross sectional area, wherein the pivoting direction is determined by the relative magnitudes of the control signals across the two valve inputs.

Yet another object of the present invention is to provide a valve of the above character which is readily adaptable to a wide range of applications requiring varying cooling rates.

These and other objects, features and advantages of the present invention will be more readily apparent after reading the following detailed description with reference to the accompanying drawings wherein:

FIG. 1 is a graphical comparison of the cooling characteristic of the present invention as compared with the prior art;

FIG. 2 is an overall system diagram showing the connection of the thermostatic expansion valve of the present invention between the compressor-condenserreceiver assembly on the one hand and the expansion coil on the other;

FIG. 3 is a cross section view of the slotted ball valve of the present invention according to a preferred embodiment illustrating its internal construction;

FIGS. 4a and 4b are enlarged partial cross sections of the slotted ball and seat of the valve of FIG. 3 showing the ball in fully closed and fully open position;

FIG. is a transverse cross section of the slotted ball of FIG. 4 taken along line 5-5 to illustrate the generally triangular flow orifice profile;

FIG. 6 is a transverse cross section of the slotted ball of the present invention according to another embodirnent wherein the slot produces a generally rectangular flow orifice profile; and

FIG. 7 is a transverse cross section of the slotted ball of the present invention according to another embodiment wherein the slot produces a chordal segment flow orifice profile.

Referring now to the drawings wherein like numerals in the different views designate the same element, the thermostatic expansion valve of the present invention is generally shown at 10. As seen in FIG. 2, valve 10 is connected between an evaporator unit 12 on the one hand and a compressor 14, condenser 16 and receiver or liquid reservoir 18 on the other. Evaporator unit 12 includes a plurality of expansion cooling coils 12a through 12f, the ends of which communicate with a common accumulator section 13. Liquid supply line 20 connects the outlet of receiver 18 and the valve inlet at 22. The outlet of receiver 18 is disposed at a slightly higher level than the bottom of the tank to trap sediment therein and prevent clogging of the valve. The valve outlet at 24 is connected to the inlet of evaporator accumulator section 13 by expansion line 26 and the outlet of accumulator 13 is connected to the suction side of compressor 14 by suction line 28.

Compressor 14 withdraws a suitable refrigerant such as Freon-l2 in vapor form from accumulator 13 through suction line 28 and delivers high pressure vapor to condenser 16. The refrigerant vapor undergoes a phase change to liquid state within condenser 16, giving up heat to the surroundings through the walls of the condenser coils. Liquid refrigerant under pressure is then collected by receiver 18 and thereafter fed to valve inlet 22 through supply line 20. When valve 10 is actuated by a suitable pressure differential sensed across its two control inputs 30 and 32, it admits controlled quantities of liquid refrigerant into valve expansion chamber 34 which communicates with evaporator 12 via expansion line 26.

Energization of compressor 14 causes expansion chamber 34, evaporator unit 12 and suction line 28 to experience a relatively lower pressure than liquid supply line 20. Thus, when valve 10 opens to allow a flow of liquid refrigerant into chamber 34, the refrigerant expands and evaporates, absorbing heat from the surroundings through the wall of cooling coils 12a through 12f.

Actuation of expansion valve 10 is dependent on the relative magnitude of the pressures applied to control inputs 30 and 32. Control input 30 is connected to gas filled thermostatic bulb 36 by pressure sensing line 38. Bulb 36 is charged with suitable refrigerant such as Freon-l2 which is in partial vapor form at room temperature and the bulb is situated in thermal contact with cooling coil 12f at the outlet end of evaporator unit 12 which is typically the coldest portion of the unit.

Control input 32 communicates via pressure sensing line 40 with evaporator unit 12 at point X which is preferably selected to be approximately three quarters along the length of evaporator 12 from inlet to outlet.

This particular point is chosen to avoid coil flooding as will be discussed hereinafter.

Referring now to FIG. 3, the internal structure and operation of the thermostatic expansion valve of the present invention will now be described. Supply line 20 is connected to the valve inlet at 22 by attachment to threaded inlet fitting 42. Similarly, expansion line 26 communicates with the interior of chamber 34 by attachment to threaded outlet fitting 44.

Valve 10 generally comprises four subassemblies designated in FIG. 2 as A, B, C and D.

Subassembly A houses the refrigerant metering mechanism and includes the following structural components. Closure plate 46 threadably receives inlet fitting 42 within a central aperture 47. Cylindrical spring housing 48 includes an exterior flange portion 48a which is attached to the interior face of closure plate 46 and an interior flange portion 48b which is attached to the exterior face of valve seat 50. Spring housing 48 is formed with a central passageway 49 which communicates with the interior of inlet fitting 42. Spring retainer 52 is disposed within passageway 49 by threadable engagement with the interior of housing 48 and includes a central aperture 53 which forms a flow orifice communicating with the interior of inlet fitting 42 on the one hand and passageway 49 on the other. The interior face of spring retainer 52 forms a shoulder which contacts the exterior end of coil compression spring 54. The interior end of spring 54 contacts slotted ball 56 and urges it against seat 50.

Seat 50 is formed with a central passageway 5 l defining a flow conduit between passageway 49 and expansion chamber 34. The exterior face of seat 50 is formed with a semi-spherical concave annular surface 50a encircling the periphery of seat orifice 51. Seat surface 50a is shaped to mutually conform to the surface of ball 56 and spring 54 urges ball 56 against surface 50a in fluid tight contact.

When ball 56 is rotated within seat 50 to an open position, slot 57 is brought into mutual registration with passageway 49 and seat orifice 51. In this orientation, liquid refrigerant introduced into inlet fitting 42 will flow through spring retainer orifice 53, passageway 49, ball slot 57, seat orifice 51 and ultimately into chamber 34. The seating force exerted on ball 56 by spring 54 is adjustable by axial movement of spring retainer 52 within housing 48.

Subassembly B forms the housing for chamber 34 which contains the linkage mechanism for pivoting ball 56 in seat 50 in response to the relative pressure differential across the two valve control inputs. Cylindrical chamber housing 58 is formed with a recessed flat surface 58a at one side to which the interior face of valve seat 50 is attached. Recessed surface 58a includes a central aperture which connects seat orifice 51 with chamber 34. The other side of housing 58 threadably receives outlet fitting 44 which also communicates with chamber 34. Closure plates 60 and 62 are fixed to the opposite outboard ends of housing 58 and each include a central aperture communicating with chamber 34.

Subassemblies C and D are structurally similar, each forming a bellows housing attached to housing 58 at opposite outboard ends of chamber 34. Subassembly C houses a bellows 64 which transmits to the ball valve a pivotal force representative of the pressure within evaporator unit 12 as monitored through sensing line 40 and applied to control input 32. Similarly, subassembly D houses a bellows 66 which transmits to the valve an opposite pivotal force which counteracts that from bellows 64. The pivotal force from bellows 66 is directly proportional to the temperature of thermal bulb 36 as indicated by its internal gas pressure monitored through sensing line 38 and applied to control input 30.

Subassembly C includes bellows housing 68 having an inboard flange 68a attached to the outer surface of closure plate 60 and an outboard flange 68b attached to the inner surface of end ring 70. Bellows 64 is sealed at its exterior margin to the inner surface of end ring 70 and at its interior margin to bellows head plate 72. Co]- lar 74 is attached to the outboard side of end ring 70 and includes a radial passageway 75 which receives pressure sensing line 40 and communicates with the interior of bellows 64 through end ring 70. Flanged end cap 76 is attached to the outboard surface of collar 74 and threadably receives end plug 78. Adjusting stem 80 threadably engages end plug 78 and passes therethrough into the interior of bellows 64 terminating in stem head 80a. Packing 79 is provided between end cap 76 and end plug 78 to maintain a pressure seal within bellows 64. Threadable movement of adjusting stem 80 within end plug 78 varies the compression of spring 82 and controls the amount of mechanical force exerted on bellows head plate 72 by spring 82 in the direction of chamber 34.

Referring now to subassembly D, bellows 66 is contained within housing 84 having an inboard flange 84a attached to the outer surface of closure plate 62 and an outboard flange 84b attached to the inner surface of end plate 86. Bellows 66 is sealed at its exterior margin to the inner surface of end plate 86 and at its interior margin to bellows head plate 88. Fitting 90 passes through end plate 86, extending into the interior of bellows 66. Fitting 90 is secured to end plate 86 by threadably engaging a nut 92 mounted to the interior surface of end plate 86 in registration with its aperture. Fitting 90 receives pressure sensing line 38 from thermal bulb 36 and causes the gas pressure exerted thereby t5 expand bellows 66 in a direction toward chamber 34 with increasing thermal bulb temperature.

Rod 94 is attached at one end to head plate 72 and at its other end to head plate 88 and forms a solid mechanical linkage joining bellows 64 and 66. The reciprocal movement of rod 94 within chamber 34 is determined by the relative magnitudes of the force exerted on head plate 72 by the combined force of spring 82 and the pressure within bellows 64 on the one hand and the force exerted on head plate 88 by the pressure within bellows 66 on the other. For example, referring to FIG. 3, if the pressure within thermal bulb 36 is sufficiently greater than the pressure within evaporator unit 12 at point X, then rod 92 will move to the right. Conversely, if the coil pressure exceeds the bulb pressure, the linkage will move to the left. The reciprocal movement of rod 92 within chamber 34 is restrained to a linear path by guide rings 96 and 98 attached to the outer surface of chamber closure plates 60 and 62, respectively.

Bellows 100 and 102 permit movement of the end portions of rod 92 into and out of chamber 34 while maintaining a pressure seal between the chamber and the bellows housing at either outboard end of the chamber. The exterior margin of bellows 100 is sealed to the interior surface of closure plate 60 and the interior margin of bellows 100 is sealed to flange 940 on rod 94. Similarly, the exterior margin of bellows 102 is sealed to the inner surface of closure plate 62 and the interior margin of bellows 102 is sealed to flange 92b on rod 92. Pressure equalizing line 104 connects the interiors of bellows 100 and 102 and the areas within bellows housings 68 and 84 exterior of bellows 64 and 66 to eliminate any undesirable false action on the valve caused by a pressure differential between these areas.

Pin 106 extends radially from rod 92 midway along its length within chamber 34. Yoke 108 engages pin 106 and moves the free end of pivot rod 110 in response to the reciprocal movement of rod 92. The opposite end of pivot rod 110 is attached to slotted ball 56 and thus yoke 108 and rod 110 translate linear reciprocal movement of rod 94 within chamber 34 into rotational movement of slotted ball 56 within its seat 50. As stated above, the pivotal rotation of ball 56 causes slot 57 to present a flow orifice of gradually varying cross-sectional area between passageway 49 and chamber 34, varying through a range from zero to the full transverse cross-sectional area of the slot.

Referring now to FIGS. 4a and 4b, the means by which the pivotal rotation of ball 56 causes slot 57 to present a flow orifice of gradually varying cross sectional profile will be seen. FIG. 4a depicts the valve in fully closed position and FIG. 4b in fully open position. As the ball is pivotally rotated within seat 50 and moved from the orientation in FIG. 4a to that of FIG. 4b, the flow orifice defined by that portion of slot 57 profile which is mutually exposed to both passageway 49 and expansion chamber 34 and interconnects the two gradually increases from zero to a pinhole opening to the full transverse section of the slot as shown in FIG. 5. The term transverse section as used herein refers to a diametrical plane perpendicular to the bottom of the slot. In the preferred embodiment shown in FIGS. 4a and 4b, slot 57 is formed by removing a wedge-shaped segment from ball 56 which produces a flow orifice cross section of roughly triangular, or more precisely sector, shape as shown in FIGS. However, the present invention is not limited to this particular slot configuration and contemplates other slot shapes. For example, the slot may have parallel sidewalls and thus forma substantially rectangular transverse section as in FIG. 6. Alternatively, the slot may be formed by removing a planar slice from the ball exterior which produces a chordal segment transverse section as shown in FIG. 7.

The operation of the valve in response to the pressure variations sensed at its control inputs will not be described. At start-up with the entire system at an ambient temperature of about 70 F., the gas pressure within thermal bulb 36 transmitted to the interior of bellows 66 exerts a force on head plate 88 moving it toward expansion chamber 34 until its movement is stopped by guide ring 98. In the embodiment shown, this distance is typically about three-eighths inch. The expansion of bellows 66 is transmitted through head plate 88 to rod 92 which moves in a direction transverse to the refrigerant flow axis of the valve (a line passing through the center of inlet fitting 42 and outlet fitting 44). This movement of rod 92 moves the free end of pivot rod 110 three-eighths inch and pivotally rotates ball 56 within seat 50 to its fully open position as in FIG. 4b. The valve opens fully at start-up since at this time there is insufficient pressure built up within evaporator unit 12 to exert an effective counterbalancing force on rod 92.

As the refrigerant flows into evaporator unit 12 and vaporizes, the internal coil pressure starts to rise. When the compressor is energized, it begins to pump off refrigerant vapor from evaporator 12 through suction line 28 thus tending to inhibit the pressure buildup within the coil. However, since the valve is at full open position, the refrigerant inflow to the coil will exceed the suction outflow and the coil pressure transmitted to the interior of bellows 64 will reach a value sufficient to exert a force on rod 92 counteracting that of bellows 66 and the valve will begin to close.

By this time, evaporator 12 is nearing the desired three-quarters level (point X) and thermal bulb 36 has started to cool decreasing the pressure on bellows 66 to rear suction pressure and aiding spring 82 in the closing of the valve. The combined action of the two control pressures prevents coil flooding of refrigerant past point X. As the evaporator temperature decreases to the desired operating level, the valve can be calibrated by adjustment of spring 82 through adjusting stem 80. Stem 80 controls the bias pressure of spring 82 against bellows head plate 72 and disposes rod 92 in a reference position with respect to the steady state operating pressure differential across the two valve control inputs 30 and 32 and allows calibration of the valve to varying coolant flow requirements.

As the ambient temperature reaches the desired level, the compressor will shut off causing a further rise in evaporator pressure and further closing flow orifice 51. At the same time, the thermal bulb has cooled to a point where its gas pressure no longer opposes the closing force on the valve and the valve is positively moved to fully closed position of FlG. 4a.

As more cooling is called for, the next cycle begins as the compressor starts, and the suction line and coil pressure rapidly drops allowing the increased pressure produced within thermal bulb 36 by the rise in coil temperature and transmitted to bellows 66 to counteract the decreased coil pressure felt within bellows 64. The result of this pressure differential is a positive reopening of the valve causing coil temperature to drop and coil pressure to rise until the pressure differential balance across the valve control inputs has shifted to once more close the valve.

An important advantage of the construction of the present invention arises from the fact that the slotted ball 57 may be easily removed from the valve and replaced with another having a different flow orifice profile shape or area. This permits use of the same basic valve in a variety of cooling applications requiring a range of refrigerant flow rates.

What is claimed is:

1. in a refrigeration system of the vapor-compression type wherein an expansion valve admits controlled quantities of refrigerant introduced at its inlet in liquid form under pressure into an expansion chamber communicating with an evaporator unit at substantially lower pressure, the improvement comprising:

a first bellows power element responsive to the internal pressure within said evaporator such that increasing evaporator pressure causes said bellows to expand;

a second bellows power element responsive to a gas pressure whose magnitude is directly proportional to the external temperature of said evaporator such that increasing evaporator temperature causes said bellows to expand;

linkage means rigidly connecting the expansion ends of said first and second bellows so that the expansion of each of said bellows opposes the expansion of the other of said bellows, said linkage being restrained to reciprocal movement in a linear path between said first and second bellows; and

a slotted ball and seat assembly wherein said ball is urged against said seat and conforms thereto in fluid tight contact, and wherein said ball is connected to said linkage means for pivotal rotation within said seat in response to the movement of said linkage to cause said slot to present a flow orifice profile between said valve inlet and said expansion chamber of gradually varying area as said ball is pivoted by said linkage.

2. The system of claim 1 wherein said first and second bellows are positioned at opposite ends of said expansion chamber and said linkage means extends through said expansion chamber perpendicular to the flow of said refrigerant through said valve.

3. The system of claim 1 wherein said seat includes a passageway extending therethrough to define a flow conduit and said passageway is encircled at one end by a semi-spherical concave annular edge on said seat, said edge being formed to mutually conform to the surface of said ball which is urged against it in fluid tight contact.

4. The system of claim 1 wherein said slot is wedgeshaped and presents a flow orifice profile of sector shape whose area varies through a range from zero to the full transverse cross sectional area of said slot as said ball is pivoted.

5. The system of claim 1 wherein said gas pressure is produced within a thermostatic bulb in contact with the exterior surface of said evaporator.

6. The system of claim 1 wherein said evaporator internal pressure is sensed at a point substantially three quarters of the distance along the length of the evaporator from its inlet to its outlet.

7. The system of claim I further comprising calibration means associated with said valve for balancing the refrigerant inflow through said inlet at a desired operating level to the refrigerant outflow from said evaporator by applying a mechanical bias against one of said bellows to aid it in opposing the pressure within the other of said bellows.

8. A refrigeration system comprising:

a source of liquid refrigerant under pressure;

an evaporator coil cooling unit maintained at a substantially lower pressure than said liquid refrigerant source;

expansion valve means having an inlet connected to said source of liquid refrigerant, and an expansion chamber connected at an outlet to said evaporator, and having two control inputs sensing external pressures,

said valve means including:

actuator means responsive to the net pressure differential across said control inputs and restrained to reciprocal movement in a linear path;

a seat element having a passageway extending therethrough to define a flow conduit, said seat being mounted between said inlet and said expanl sion chamber so that said flow conduit lies therebetween, said seat having a semi-spherical concave annular face portion extending around the periphery of said passageway at one end surface of said seat element;

a slotted ball held in urged contact against said face and conforming thereto in fluid tight relationship, said ball being mounted for pivotal rotation within said seat and having a slot formed therein to present a flow orifice connecting said inlet and said flow conduit, said orifice exposing a gradually varying profile area ranging from zero to the full transverse cross sectional area of said slot and said ball is pivoted within said seat; and

linkage means connecting said actuator means and said ball for pivotal rotation of said ball within said seat in response to reciprocal linear movement of said actuator means.

9. The process of metering the flow of liquid refrigerant from a valve inlet to a valve expansion chamber communicating through the valve outlet with an evaporator cooling coil, comprising the steps of:

sensing the pressure within said coil;

sensing a gas pressure whose value is directly proportional to the exterior temperature of said coil;

applying each of said pressures to opposite ends of a rigid linkage element to cause said linkage to move reciprocally in a linear path in response to the greater of said pressures; and

pivoting a slotted ball in urged fluid tight contact with a semi-spherical valve seat in response to the linear movement of said linkage to cause said slot to present a flow orifice of gradually varying profile area communicating said inlet with said expansion chamber.

10. The process of claim 9 wherein the evaporator pressure is sensed by transmitting it to the interior of a first sealed bellows, and the gas-pressure is sensed by transmitting it to the interior of a second sealed bellows, and said pressures are applied to said linkage by connecting the expansion ends of said first and second bellows to opposite ends of a shaft restrained to reciprocal movement along a linear path.

11. The process of claim 9 further comprising the step of balancing the refrigerant in-flow through the valve inlet to the refrigerant out-flow leaving the evaporator at a desired operating level by applying a mechanical spring force to one of said bellows to aid the pressure within that bellows in counterbalancing the pressure within the other bellows. 

1. In a refrigeration system of the vapor-compression type wherein an expansion valve admits controlled quantities of refrigerant introduced at its inlet in liquid form under pressure into an expansion chamber communicating with an evaporator unit at substantially lower pressure, the improvement comprising: a first bellows power element responsive to the internal pressure within said evaporator such that increasing evaporator pressure causes said bellows to expand; a second bellows power element responsive to a gas pressure whose magnitude is directly proportional to the external temperature of said evaporator such that increasing evaporator temperature causes said bellows to expand; linkage means rigidly connecting the expansion ends of said first and second bellows so that the expansion of each of said bellows opposes the expansion of the other of said bellows, said linkage being restrained to reciprocal movement in a linear path between said first and second bellows; and a slotted ball and seat assembly wherein said ball is urged against said seat and conforms thereto in fluid tight contact, and wherein said ball is connected to said linkage means for pivotal rotation within said seat in response to the movement of said linkage to cause said slot to present a flow orifice profile between said valve inlet and said expansion chamber of gradually varying area as said ball is pivoted by said linkage.
 2. The system of claim 1 wherein said first and second bellows are positioned at opposite ends of said expansion chamber and said linkage means extends through said expansion chamber perpendicular to the flow of said refrigerant through said valve.
 3. The system of claim 1 wherein said seat includes a passageway extending therethrough to define a flow conduit and said passageway is encircled at one end by a semi-spherical concave annular edge on said seat, said edge being formed to mutually conform to the surface of said ball which is urged against it in fluid tight contact.
 4. The system of claim 1 wherein said slot is wedge-shaped and presents a flow orifice profile of sector shape whose area varies through a range from zero to the full transverse cross sectional area of said slot as said ball is pivoted.
 5. The system of claim 1 wherein said gas pressure is produced within a thermostatic bulb in contact with the exterior surface of said evaporator.
 6. The system of claim 1 wherein said evaporator internal pressure is sensed at a point substantially three-quarters of the distance along the length of the evaporator from its inlet to its outlet.
 7. The system of claim 1 further comprising calibration means associated with said valve for balancing the refrigerant inflow through said inlet at a desired operating level to the refrigerant outflow from said evaporator by applying a mechanical bias against one of said bellows to aid it in opposing the pressure within the other of said bellows.
 8. A refrigeration system comprising: a source of liquid refrigerant under pressure; an evaporator coil cooling unit maintained at a substantially lower pressure than said liquid refrigerant source; expansion valve Means having an inlet connected to said source of liquid refrigerant, and an expansion chamber connected at an outlet to said evaporator, and having two control inputs sensing external pressures, said valve means including: actuator means responsive to the net pressure differential across said control inputs and restrained to reciprocal movement in a linear path; a seat element having a passageway extending therethrough to define a flow conduit, said seat being mounted between said inlet and said expansion chamber so that said flow conduit lies therebetween, said seat having a semi-spherical concave annular face portion extending around the periphery of said passageway at one end surface of said seat element; a slotted ball held in urged contact against said face and conforming thereto in fluid tight relationship, said ball being mounted for pivotal rotation within said seat and having a slot formed therein to present a flow orifice connecting said inlet and said flow conduit, said orifice exposing a gradually varying profile area ranging from zero to the full transverse cross sectional area of said slot and said ball is pivoted within said seat; and linkage means connecting said actuator means and said ball for pivotal rotation of said ball within said seat in response to reciprocal linear movement of said actuator means.
 9. The process of metering the flow of liquid refrigerant from a valve inlet to a valve expansion chamber communicating through the valve outlet with an evaporator cooling coil, comprising the steps of: sensing the pressure within said coil; sensing a gas pressure whose value is directly proportional to the exterior temperature of said coil; applying each of said pressures to opposite ends of a rigid linkage element to cause said linkage to move reciprocally in a linear path in response to the greater of said pressures; and pivoting a slotted ball in urged fluid tight contact with a semi-spherical valve seat in response to the linear movement of said linkage to cause said slot to present a flow orifice of gradually varying profile area communicating said inlet with said expansion chamber.
 10. The process of claim 9 wherein the evaporator pressure is sensed by transmitting it to the interior of a first sealed bellows, and the gas-pressure is sensed by transmitting it to the interior of a second sealed bellows, and said pressures are applied to said linkage by connecting the expansion ends of said first and second bellows to opposite ends of a shaft restrained to reciprocal movement along a linear path.
 11. The process of claim 9 further comprising the step of balancing the refrigerant in-flow through the valve inlet to the refrigerant out-flow leaving the evaporator at a desired operating level by applying a mechanical spring force to one of said bellows to aid the pressure within that bellows in counterbalancing the pressure within the other bellows. 